New experimental data are reported for equilibrium adsorption isotherm and mass transfer of pure argon on a sample of pelletized lithium-exchanged low silica X (LiLSX) zeolite. Model analysis of the data indicate that the zeolite behaved like a nearly homogeneous adsorbent for Ar adsorption while it exhibited substantial heterogeneity for adsorption of N-2 and milder heterogeneity for adsorption of O-2. The overall mass transfer coefficient for Ar adsorption was comparable in magnitude with those of N-2 and O-2. The coefficient increased with increasing pressure and decreased with increasing temperature like those for N-2 and O-2. A large skin resistance at the adsorbent particle surface was observed for Ar mass transfer like that for adsorption of O-2 and N-2.